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In this sense the word may be defined as that vital function by which an organism changes nutrient material into living protoplasm.
Most modern scientists admit that the notion of assimilation is not exhausted by the eventual chemical changes that may take place. Their definition of assimilation, moreover, is most frequently the true expression of reality. To give but one instance, the physiologist Rosenthal defines assimilation as the "peculiar property common to all cells of bringing forth from different materials substances specifically similar to those which pre-exist in those cells". But, in further explaining the concept of assimilation, they frequently mistake its true nature and deny again what they conceded before. In other words, they often refuse to acknowledge that food, in being changed into living substance, participates in properties which in themselves are of a nature totally different from the forces of inorganic matter.
Our reason for disapproving this view rests on the fact that, while the action of inorganic matter is essentially of a transient nature, and passes from subject to subject, the same inanimate matter acquires by the process of assimilation the faculty "of acting on itself, of developing and perfecting itself by its own motion, or of acting immanently". That is, the action proceeds from an internal principle and "does not pass into a foreign subject, but perfects the agent." The activities implies in the nutrition of an animal really proceed from it. It spontaneously moves about and selects among a thousand solid particles a definite kind and quantity of food in strict proportion to its own needs, and appropriates it in a suitable manner. Then, in anticipation of a definite end to be realized, it elaborates from the food the chemical constituents to be used for the renewal and increase of its protoplasm, rejecting the rest in a suitable manner. Thus the entire action proceeds from the animal and finally serves, or tends to serve, no other purpose than to maintain the integrity of its protoplasm and to give it the total perfection of the species. On the other hand, it is evident that such immanent actions belong to a sphere totally different from the transient actions of which alone inorganic matter is capable. If inorganic matter is to act, it must be acted upon, and the reaction is mathematically equal to the action. It is, therefore, merely passive. But organisms act, even if no action is exerted upon them from without; and if an action results from stimulation, the reaction is not equal to the action, nor is, in fact, the stimulation the adequate cause of the action. In this activity, however, we need not assume a production and accumulation of new material energy. The activity of the vital principle in the processes of assimilation simply consists in directing the constant transformation of existing material energy towards definite ends and according to a definite plan of organization. In other words, the algebraic sum of all the energy in the universe is not altered by the living principle. Nor are the elements changed in their nature and mutual action. They require the faculty of an immanent action merely inasmuch as they are and remain parts of living cells. Thus, through assimilation they become subject to a higher principle which in constant agreement with their own physical and chemical laws directs them towards the uniform perfection of the entire organism.
Rosenthal, Allgemeine Physiologie (1901), 392; Pesch, Institutiones psycholog icoe, Pars I, lib. I, 144; Maher, Psychology (1895), 510.